The use of antireflection coatings for optical devices such as lenses in photographic and other equipment is well known. These coatings take advantage of the relationship described in the Fresnel formulae. It is well established that the refractive index of an overlying material should be at about the square root of the refractive index of the underlying material and that the thickness of the coating or layer should be an odd integer multiple of one-quarter of the wavelength of the incident radiation, the so-called "quarter-wavelength thickness".
In U.S. Pat. No. 4,759,990 to Y-T. Yen, the antireflective coating concept is extended for use over optical membrane elements, i.e., pellicles, which may be used in the manufacture of semiconductor wafers. The pellicle material is typically a nitrocellulose film which is stretched over a pellicle holder (an upstanding ring or the like). The use of an aromatic vinyl polymer as a first coating layer such as polyvinyl naphthalene, polymethylstyrene or polystyrene, followed by a fluorocarbon layer such as 3M's FC-721 or FC-77 is disclosed. Pellicles may be used a number of times before they suffer either mechanical break down or cover contamination. Such pellicles are either cleaned or reworked/replaced.
Tanaka et al., J. Electrochem. Soc., 137, 3900 (1990) (Tanaka I) provides a technique for directly applying an antireflective coating on top of a resist (ARCOR). This technique seeks to overcome the many problems which have been encountered due to reflectivity when increasing the degree of pattern densification in trying to achieve ULSI having sub-0.5 um geometries.
Light interference leads to linewidth variations and degraded detection of through the lens (TTL) alignment marks. Tanaka I discloses the formation of an antireflective film on a resist surface which is sufficient to suppress the multiple interference effects due to repeated reflection of incident light in the resist film. The ARCOR film is clear and has its thickness and refractive index optimized and is used in a process which entails the steps of coating and baking a photoresist in the conventional way, spin-coating a film of the ARCOR material on the photoresist, imaging the composite structure, removing the ARCOR film, and developing the photoresist. The ARCOR process adds the steps of spin-coating and removal of the ARCOR film. The following materials were characterized:
______________________________________ ARCOR Material Refractive Index* dLW Factor** ______________________________________ perfluoroalkylpolyether 1.29 10X (1) polysiloxane 1.43 2.5X polyethylvinylether 1.48 1.7X (2) polyvinylalcohol 1.52 1.4X ______________________________________ *Refractive index in eline (546 nm), gline (436 nm) and iline (365 nm). **Reduction in linewidth variation due to reflectivity.
Using the Fresnel formulae, Tanaka I determined that the refractive index of the ARCOR material should be about equal to the square root of the refractive index of the imaging resist used. Tanaka I used a resist having a refractive index of 1.64 and the ideal ARCOR refractive index is 1.28. The Tanaka et al. materials fall into two categories: (1) those which suppress reflection effects and which require organic solvent strip; and (2) those which are aqueous strippable, but which provide little process benefit. (Refractive index.gtoreq.1.48.)
Tanaka et al., J. App. Phys., 67, 2617 (1990) (Tanaka II) discloses the use of perfluoroalkyl polyether or spin on glass and di-propoxy-bis(acetyl-acetonate)titanium as ARCOR materials for single and bilayer films to control the interface reflectivities as well as the methods to measure such reflectivities. Tanaka II entails the use of baking steps to fix the one and two layer ARCORs disclosed. It is silent as to post exposure and develop methods, if any, to remove the ARCOR layer(s).
Tanaka et al., Chem. Abs 107:87208y (1987) (Tanaka III) is directed to the subject matter of JP 62 62,520, viz., a process for coating a photoresist with an antireflective coating which may comprise perfluoroalkyl-polyether, perfluoroalkylamine, or perfluoroalkyl-polyether-perfluoroalkylamine mixed film. The reflection-preventive film is removed after pattern wise exposure using a Freon (a chlorofluorocarbon compound) solvent.
The Tanaka materials having the desired refractive indices are more expensive to use. First, there is the required additional process step to remove the ACOR material. Second, the removal requires an organic solvent which is expensive to make/purchase and requires expense to safely handle and dispose of. Third, the nature of Tanaka's solvents as CFCs requires extreme care to protect against environmental damage. The waste management aspects of the Tanaka reflection-preventative materials weighs heavily against their implementation.
In U.S. Pat. No. 4,701,390 to Grunwald et al., a process for thermally stabilizing a photoresist image layer formed on a substrate is provided, wherein the image layer, prior to being sujected to a post-development bake, is coated with a protective material which bonds to the photoresist, but is readily rinsed from the exposed substrate after post bake and which does not interfere with the desired operation of any of the subsequent steps of pattern generation including final removal of the photoresist image. The protective (thermally stabilizing) material may be a compound or a mixture of two or more compounds selected from chromotropic acid, perfluorocarbon carboxylic acids, perfluorocarbon sulfonic acids, perfluorocarbon phosphoric acids, and alkali metal, ammonium and amine salts of such acids, ethoxylated perfluorocarbon alcohols, and quaternary ammonium salts of N-perfluoro-N',N"-dialkylamines.